Battlefield was the first story in season twenty-six of Doctor Who, which would ultimately be the last season of the original 1963-89 series.

Nicholas Courtney made his first appearance as Brigadier Lethbridge-Stewart since 1983's The Five Doctors, and Bessie appeared for the first time since that serial as well. Following Courtney's death in 2011, this serial marked his last televised performance as Lethbridge-Stewart in the series proper; that same year, his character was written out of the series in The Wedding of River Song out of respect for the actor's passing. However, Lethbridge-Stewart as a character would return to the series approximately three years later, making a minor appearance near the end of Death in Heaven in a brief scene paying tribute to the late Courtney.

This was also the first story to feature UNIT since The Seeds of Doom (bar a brief appearance in The Five Doctors) and also the last of the classic series — UNIT would be featured once again in Aliens of London in 2005. Jean Marsh, who played Sara Kingdom in the 1960s, returned in a different role. The Seventh Doctor's costume was also modified as of this story: to coincide with his darkening personality, he replaced his French grey jacket and red-banded hat with dark brown equivalents.

Interestingly, Battlefield introduced the idea that the Doctor was the man behind the myth of the wizard Merlin at some point in his life. Since this ended up being one of the final stories of the classic series, the full details behind this mysterious plot line were never worked out on television and instead interpreted differently across the Doctor Who expanded universe as multiple writers tried their hand at explaining these untold events.

A battle breaks out between UNIT and Morgaine's army. Brigadier Lethbridge-Stewart has come out of retirement to assist in the crisis and ends up using silver bullets to kill the Destroyer — an awesomely powerful creature unshackled by Morgaine to devour the world — although he himself is almost killed in the process.

Morgaine tries to fire the nuclear missile but is overcome by shock when the Doctor tells her what nuclear war would really mean, and that Arthur is in fact dead. She and her son are then taken prisoner by UNIT.

Brigadier Lethbridge-Stewart, now retired from teaching (TV: Mawdryn Undead) as well as UNIT, is enjoying retirement with his wife Doris. UNIT may still need the gallant old soldier, though; new Brigadier Winifred Bambera and her soldiers, in a nuclear missile convoy near rural Carbury, are faced with inexplicable equipment failure and peculiar sightings of armour-clad knights. In the excitement, the UNIT convoy ignores a pair of hitchhikers nearby: the Seventh Doctor and Ace, lured to Carbury by a distress signal so powerful that it even crosses universes.

The Doctor and Ace eventually hitch a ride with archaeologist Peter Warmsly, who is excavating an old battlefield near Carbury — though even he is surprised to find his dig site taken over by tight-lipped UNIT soldiers. While he attempts to get an explanation, the Doctor unearths old UNIT identification (his own and Liz Shaw's) to get himself and Ace into the UNIT camp. Bambera, nonplussed, confiscates the passes and ejects the two travellers from the convoy. But one UNIT soldier, who had previously served under Lethbridge-Stewart, recalls UNIT's former scientific advisor: his eccentricity, his ability to change appearance, and his tendency to attract trouble.

Intrigued, Bambera gives the Doctor and Ace a lift to the nearby Gore Crow Hotel... and asks UNIT headquarters to recall Lethbridge-Stewart to duty.

At the hotel, Ace meets a kindred spirit in Shou Yuing, while the Doctor talks to innkeeper Elizabeth Rowlinson. She is blind but psychic, and knows that the ancient scabbard over the inn's fireplace — an artefact found by Warmsly — is an object of tremendous and mysterious power.

Ace and Shou are chatting outside the hotel when they and the Doctor witness an oddly human-shaped missile crash into innkeeper Pat Rowlinson's microbrewery. It is revealed to be a knight in full armour — not an android, as Ace initially thinks — and, when his helmet is removed, he warmly greets the Doctor as "Merlin".

The Seventh Doctor, Ace, and Shou chat with the knight, Ancelyn, who refers to the distress signal intercepted by the TARDIS as "Excalibur's call" and claims that it heralds the start of a final war and King Arthur's restoration to the throne. The Doctor surmises that the coming battle isn't properly part of this dimension at all, and belongs to a parallel universe — but before he can test the theory, an exasperated Brigadier Bambera storms into the microbrewery and declares everyone under arrest. She is interrupted by the unexpected arrival of more extradimensional knights, including Ancelyn's sworn enemy Mordred, who also recognises the Doctor as Merlin. After Mordred retreats, unwilling to face the Doctor, Ancelyn and Bambera vie for dominance and the rest of the party retreats to the Gore Crow's lounge.

Mordred, with the help of a sword he calls "brother to Excalibur", begins a strange ritual intended to link his home dimension to present-day Carbury and allow his mother Morgaine to cross over. As he roars in triumph at the ritual's success, the scabbard on the wall of the Gore Crow's lounge breaks free and sails across the room — towards the lake — before implanting itself in a wall, distressingly close to Warmsly. The Doctor realises what is happening, but cannot stop it... Morgaine crosses over, and the hotel is plunged into darkness.

Morgaine and Mordred discuss their plans for battle; the sorceress, seeing a helicopter approach, demonstrates her power by bringing it down. The helicopter, which is bringing Lethbridge-Stewart to Carbury, manages to land, and Lethbridge-Stewart sets off in search of the Doctor.

Warmsly shows the Doctor and Ace the Carbury dig, and points out a mysterious inscription. The Doctor recognises his own handwriting instructing him to dig a hole, which he asks Ace to do with some well-placed explosives. At the bottom is an 8th century tunnel... made of cement. The Doctor and Ace investigate, having left Warmsly to guard them.

At the end of the tunnel is a strange room. Ace and the Doctor look in vain for some way out, until the Doctor simply commands the door to open, and it obeys. As they proceed to the next rooms, the Doctor explains that Merlin must have built the rooms — spaceship, it now appears — that they are in... and he is Merlin, or will be.

In the topmost level of the spaceship is King Arthur, apparently in suspended animation, and Excalibur. Ace accidentally draws Excalibur, which triggers an automated defence system that traps her in an airtight chamber rapidly filling with water, and renders the Doctor helpless as she begins to drown...

As the two wander near Lake Vortigern, Warmsly quotes Malory's "Morte d'Arthur" to Ancelyn... when, to his astonishment, a hand appears from the lake brandishing Excalibur. It proves to be Ace, whom the Doctor managed to free from the spaceship below the lake. She emerges wet but none the worse for the wear, and is joined by Warmsly, Ancelyn, Bambera, and, arriving with Shou in her car, which he has commandeered, Lethbridge-Stewart. The Doctor continues to fight the spaceship's defence system, but it is only stopped when Lethbridge-Stewart, who has entered through the tunnel, intervenes. Reunited, the friends leave by the tunnel and rejoin the others at the dig.

Meanwhile, Morgaine has sent Mordred and his knights after the Doctor and the others, with orders to take Excalibur and kill any who resist. The Rowlinsons are terrified by Morgaine's cavalier killing of Flight Lieutenant Lavel, but grateful — if bewildered — when she restores Elizabeth's sight.

The party splits up for the ride back to the hotel: Bambera and Ancelyn in Shou's car; the Doctor, Ace, Shou, and Warmsly in his. Mordred's knights set up an ambush in the woods between the hotel and the dig, but ultimately attack the wrong car; the others make it back to the hotel safely, with Excalibur. There they find that all locals are being evacuated — Shou and Ace slip away unnoticed, while Warmsly and the Rowlinsons protest but ultimately agree to evacuate.

The Doctor is delighted to see Bessie again.

A UNIT patrol sent to find Bambera and Ancelyn reports that the two soldiers escaped the knights and are at large in the woods. The news, though welcome, does not entirely cheer the Doctor, who points out that Mordred and his knights are also at large... and that nothing stands between them and the now-unguarded missile convoy by the lake. They must return to the convoy immediately. Fortunately for the Doctor, Lethbridge-Stewart thought to bring along transportation: Bessie, the Doctor's car from his UNIT days. He and the Doctor depart, leaving Ace and Shou behind with Excalibur and instructions on avoiding Morgaine's sorcery.

Morgaine leaps at her chance to snatch Excalibur from the hands of its young guardians, summoning the Destroyer to help her, but Ace remembers the Doctor's warning and draws a protective circle of chalk. Much to her surprise, it seems to work... until Morgaine tries a more subtle psychological attack, and sets Ace and Shou fighting, trying to draw one or both out of the circle.

At the dig, the ancient battlefield has become a modern one, as UNIT troops face off against Mordred and his knights, and Mordred himself is confronted by Ancelyn — but as they prepare to fight, the Doctor intervenes and declares that there will be no more bloodshed. But Mordred smugly announces that the battle was only a diversion for his mother Morgaine's attempt at Excalibur... and that Ace and Shou stand no chance against Morgaine and the Destroyer.

Mordred offers the Doctor a trade — the girls' lives for Excalibur — but the Doctor, outraged, threatens to decapitate him unless Morgaine surrenders herself. But Mordred calls the Doctor's bluff; he knows, he says, of Merlin's distaste for taking life. He is proved right, but Lethbridge-Stewart appears out of the woods, gun ready, and declares that he has no such compunctions. The two old friends force Mordred into their car and head back to the Gore Crow Hotel.

They arrive just as the Destroyer, under Morgaine's command, has nearly brought the hotel down on Ace and Shou. The Doctor finds his friends under a pile of rubble, relieved that they were able to stay alive, even at the cost of surrendering Excalibur to Morgaine.

She, meanwhile, has returned, with Excalibur and Mordred, to her castle in her reality. The Doctor and the Brigadier follows her across the void; he himself is followed by Ace, who brings along Excalibur's sheath and the Brigadier's silver bullets.

Morgaine frees the Destroyer, to the Doctor's dismay. While she and Mordred teleport away, out of the Destroyer's immediate path, the Doctor and his two friends are left to flee on foot. The Doctor deduces that, since the Destroyer was bound in silver chains, it will be vulnerable to the Brigadier's silver bullets. Lethbridge-Stewart distracts the Doctor, knocks him out and takes the gun — loaded with the silver bullets — to spare the Doctor the duty of killing the Destroyer. When the Doctor revives, he and Ace find that Lethbridge-Stewart has done so, nearly paying for the opportunity with his life.

Even without the Destroyer, Morgain and Mordred still pose a threat, as they prove when they cross back into our world and take Bambera hostage. They force her to divulge the launch codes for the nuclear missile, and prepare it for launch.

Ancelyn, the Doctor, Lethbridge-Stewart, and Ace return Excalibur to the spaceship under the lake. There they find that the sword activates the spaceship — but does not resurrect Arthur. According to a note left by the Doctor for himself, Arthur was killed in the Battle of Camlann, not placed in suspended animation. The note also warns them that Morgaine has control of the nuclear missile, and the four friends rush back to the surface.

The Doctor and Ancelyn admire the Brigadier's garden.

While Ace and the Brigadier destroy the spaceship, the Doctor tries to stop the missile launch. Morgaine is obstinate... but the Doctor explains the full horror of nuclear warfare, and she relents with seconds to spare. She demands of the Doctor that King Arthur face her for a final confrontation, but the Doctor reveals that Arthur died in his last battle. Grief-stricken, she does not notice the Doctor leave.

The Doctor intervenes in a sword fight between Ancelyn and Mordred, and renders Mordred unconscious. Ancelyn is freed, and Mordred and Morgaine surrendered to UNIT.

Back at the Lethbridge-Stewart house, the Brigadier and Ancelyn are left to work in the garden while the Doctor cooks supper. In the meantime, Doris, Shou, Ace, and Bambera have commandeered Bessie for a girls' trip out on the town.

The Brigadier talks of how UNIT now has a variety of anti-alien weaponry: silver bullets, teflon anti-Dalek bullets, high explosives for Yetis, armour piercing rounds for robots, and gold-tipped bullets for "you know what" (Cybermen).

The Doctor and Ace use passes that he thought he would never need again. Her pass belonged to Liz Shaw.

This story had the working titles Knightfall, Storm Over Avallion, Lake Over Avallion, Pool of Avallion, Song of Avallion, Stormtroopers of Avallion, and The Battlefield.

This story takes place "a few years in the future" from Ace's starting point. This is made evident when Ace is surprised that the combined costs of a glass of water and a glass of lemonade are five pounds.

This is the last appearance of UNIT in the show's original run. UNIT resumed broadcast appearances in Doctor Who beginning with TV: Aliens of London.

This is the last appearance of the Brigadier in Doctor Who itself, although he would make one more televised appearance in the spin-off The Sarah Jane Adventures.

Radio Times credits Angela Bruce as "Brig. Bambera" for parts one, three and four, and as "Brigadier Bambera" for part two. All on-screen credits read "Brigadier Winifred Bambera".

June Bland (Elizabeth Rowlinson) is credited as "Elizabeth" in Radio Times.

The Radio Times programme listing for part three in certain regions was accompanied by a black and white publicity shot of the Doctor and Mordred, which showed the Doctor pushing up the visor on Mordred's helmet, with the accompanying caption "Let's just have a look at you: the time-travelling Doctor (Sylvester McCoy) unmasks Mordred (Christopher Bowen) / Doctor Who 7.35 p.m. BBC1". There was also a slightly altered synopsis, which read 'The Doctor has gone to the battlefield, leaving Ace and Shou Yuing back at the hotel. All they have to worry about there is a demon, nuclear weapons and a witch "playing games with their minds". The synopsis in other regions was the same, apart from the omission of the last few words "playing games with their minds".

During the water-tank scene at the end of part two, the tank's glass cracked, sending broken glass and water across the studio towards the electrical equipment. By chance, Sylvester McCoy noticed in time to alert the crew; Sophie Aldred was pulled out in time and the studio was hurriedly evacuated. To ensure that nobody assumed that he was ad-libbing in character, McCoy shouted obscenities before asking "someone [to] get her out of there!" It is commonly believed by cast and crew that Aldred might have died if McCoy hadn't noticed in time; Gary Downie has said she was in no danger and it was only the floor crew at risk (Doctor Who Magazine #338), although other cast/crew believed otherwise.

On the Battlefield DVD, camera image of part of the incident can be seen.

Years later, McCoy would remember that day as the one time he himself was the hero instead of the Seventh Doctor.

This is the last serial in the classic series to feature the TARDIS console room, as well as being the only TARDIS interior scene of Season 26. The sequence in part one, where the Doctor and Ace receive a mysterious distress signal from sideways in Time, was recorded on a hastily contructed set — the shortcomings of which were disguised by having the console room in semi-darkness — as the regular TARDIS scenery wall flats had been mistakenly junked after recording of The Greatest Show in the Galaxy.

As well as featuring foreign UNIT troops (from France, Hungary and Poland), Battlefield does a soft retcon by stating (via Zbrigniev's service under Lethbridge-Stewart) that there had been foreign troops "off-screen" in the Pertwee era. This was an intentional act by Aaronovitch to emphasise UNIT was an international group, confirmed on the Battlefield DVD's behind-the-scenes features.

In the expanded universe, Alistair Lethbridge-Stewart reappears multiple times, starting with No Future. The audio drama Minuet in Hell established that the United Nations would "dust off" Lethbridge-Stewart as an agent several years after Battlefield. The Brigadier also appears in the independent film production Downtime.

The Doctor is shown to be capable of hypnotising people with only his eyes (TV: The Hand of Fear).

The Seventh Doctor meets Lethbridge-Stewart for the "first time" (from the Brigadier's perspective) in PROSE: No Future (and then wipes his memory of the event so he could meet him for the first time in this story).

The Doctor appears to be aware of the circumstances of the Brigadier's eventual death, stating that he is "supposed to die in bed". In his eleventh incarnation, the Doctor is told by a nurse that the Brigadier did in fact die peacefully in bed. (TV: The Wedding of River Song)

The Doctor utilises the phrase "When I say 'run,' run... run!" during his confrontation with the spaceship's defense system; this phrase was repeatedly used by his second incarnation as a response to danger.